Even though you have all the time in the world to plan your next move, sci-fi tactics game XCOM: Enemy Unknown still manages to be a nail-bitingly tense experience.

That's because one wrong move can practically doom your entire mission to eradicate a cell of aliens, deactivate a time bomb, or escort a foreign diplomat to safety.

One silly misstep could see a soldier dashing into a room full of ape-like Muton chargers, or outflanked by floating robot torsos, or in the sightlines of a bulbous-brained telepathic alien who can turn your medic into a mindless zombie.